1. Field of the Invention
The present invention relates generally to photodetectors and particularly to a charging current integrating type photodetector which can provide a highly linear output relative to its light input intensity.
2. Description of the Prior Art
A photodiode charge storage type photodetector has hitherto been available as a photodetector directed to high speed detection of photon flux. Such a photodetector is disclosed in "Operation of p-n Junction Photodetectors in a Photon Flux Integrating Mode" by Gene P. Weckler, IEEE Journal of Solid-State Circuits, Vol. SC-2, No. 3, September 1967, pp 65-73.
Reference is made to FIG. 1 which is useful to explain the principle of this known photodetector. A switch 1 is turned on at time t.sub.o and a photodiode 2 is charged up to a voltage Vo of a voltage source 3. The photodiode has a junction capacitance C which can be expressed in general by equation (1), EQU C=Co(V+Vd).sup.-n ( 1)
where V is a reverse bias voltage, Vd a diffusion potential, and n a constant which varies within a range of 1/2 to 1/3 in accordance with the junction concentration distribution.
When charged with the voltage Vo, the photodiode has a charge Q.sub.Vo which is given by equation (2), ##EQU1## Equation (2) is then reduced to, EQU (Co/(1-n))[(Vo+Vd).sup.(1-n) -Vd.sup.(1-n) ] (3)
When the photodiode receives the photon flux illumination and a photoelectric current Ip begins to flow through the photodiode pn junction, the charge stored in the junction capacitor is caused to discharge. During time lapse t.sub.s counting from the open-circuit of the switch 1, the stored charge discharges by an amount Qp which is, ##EQU2## where .alpha. is the sensitivity of the photodiode, p the light input intensity, and S the light receiving area.
It is now assumed that a charged voltage which remains across the photodiode at the termination of the time lapse t.sub.s is denoted by Vt, then equation (6) stands: ##EQU3## From equations (3), (5) and (6), ##EQU4## is obtained which is reduced to, ##EQU5##
Accordingly, a voltage change .DELTA.V which is due to decrement in the charged voltage of photodiode caused by conduction of the photoelectric current is given by, ##EQU6##
The switch 1 is again turned on at the termination of the time lapse t.sub.s so that recharging current is passed into the junction capacitor of the photodiode 2, producing an output voltage Vout across an output resistor 4. The output voltage, decreasing exponentially, has a peak value which is equal to .DELTA.V.
Conventionally, the peak value of the output voltage Vout is detected to measure the intensity of the photon flux illumination. However, this peak value depends on Vo, Vd and n as will be seen from equation (10) and fails to make itself duly proportional to the product of incident photon flux intensity p and charge storing time t.sub.s, giving rise to bad linearity between light input intensity and output voltage.